1. Field of the Invention
This invention relates to the preparation of .alpha.,.beta. -unsaturated aldehydes. More particularly, this invention relates to a method for preparing a catalyst characterized in that ammonium rhodanide is added during the stage of preparing the catalyst. Especially, this invention provides a catalyst having an enhanced activity for producing .alpha.,.beta. -unsaturated aldehydes by the gas phase catalytic oxidation of .alpha.,.beta. -unsaturated hydrocarbons.
2. Description of the Prior Art
Metal oxide catalysts comprising as main metal components molybdenum, bismuth and a transition metal, especially iron, cobalt or nickel, optionally containing other elements incorporated according to need, have heretofore been used as catalysts for the gas phase catalytic oxidation of .alpha.,.beta. -unsaturated hydrocarbons. However, the known catalysts of this type have various disadvantages.
For example, the catalyst disclosed in Japanese Patent Publication No. 4771/69 (oxide catalyst of the Mo-Bi-Fe-As system) and the catalyst disclosed in Japanese Patent Publication No. 6246/69 (oxide catalyst of the Mo-Bi-Co-Ni-Fe-As system) include a poisonous substance such as arsenic oxide as a component which is effective for obtaining the desired acrolein product in a high yield. Accordingly, when these catalysts are employed, there is a danger that poisonous substances such as arsenic will be discharged outside the reaction system, depending on the working conditions or the working procedures. Further, since arsenic compounds have a very high toxicity, such operations as the preparing of an arsenic compound-containing catalyst, charging of such catalyst into a reaction vessel and withdrawal of the catalyst from a reaction vessel are very dangerous. Therefore, the method for preparing acrolein on an industrial scale by using a catalyst containing a poisonous substance such as an arsenic compound cannot be regarded as being an optimum method for industrial usage.
When oxides of Mo, Bi and Fe are used as main components of such a catalyst, there is generally adopted a catalyst-preparing method comprising dissolving ammonium molybdate in water, forming a nitric acid-acidified aqueous solution of ferric nitrate and bismuth nitrate, mixing both the solutions, adding a suitable carrier and other salts to the mixture according to need and performing the concentration, drying, molding and calcination steps. However, an oxide catalyst of the MoBi-Fe system prepared by this method possesses a much lower activity than an oxide catalyst of the Mo-Bi-Fe-As system, and it is not an advantageous catalyst for preparing acrolein economically. Further, during the preparation of this catalyst, molybdenum reacts with iron to form a gel-like precipitate and a good dispersion state of the metals cannot be obtained. Accordingly, it is difficult to obtain good reproducible results of catalytic activity when separate batches of catalyst are prepared. For these reasons, the use of arsenic in the oxide catalyst of this type is advantageous in terms of yield, but the use of arsenic is not preferred for the reasons set forth above.